Obstructive sleep apnea syndrome (OSAS) is a well recognized disorder which may affect as much as 1-5% of the adult population. OSAS is one of the most common causes of excessive daytime somnolence. OSAS is most frequent in obese males, and it is the single most frequent reason for referral to sleep disorder clinics.
OSAS is associated with many conditions in which there is an anatomic or functional narrowing of the patient's upper airway, and is characterized by an intermittent obstruction of the upper airway occurring during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia and sleep disruption associated with the apneas and subsequent arousals from sleep.
The pathophysiology of OSAS has not yet been fully worked out. However, it is well recognized that obstruction of the upper airway during sleep is in part due to the collapsible behavior of the supraglottic segment of the airway resulting from negative intraluminal pressure generated by inspiratory effort. Thus, in patients suffering from OSAS, the upper airway during sleep behaves substantially as a Starling resistor (i.e., the airflow is limited to a fixed value irrespective of the driving (inspiratory) pressure). Partial or complete airway collapse may then occur with the loss of airway tone which is characteristic of the onset of sleep and which may be exaggerated in OSAS.
Since 1981, positive airway pressure (PAP) therapy applied by a tight fitting nasal mask worn during sleep has evolved as the most effective treatment for OSAS, and is now the standard of care. The availability of this non-invasive form of therapy has resulted in extensive publicity for OSAS and the appearance of large numbers of patients who previously may have avoided the medical establishment because of the fear of tracheostomy. Increasing the comfort of the PAP system has been a major goal of research aimed at improving patient compliance with the PAP therapy.
PAP therapy has become the mainstay of treatment in Obstructive Sleep Disordered Breathing (OSDB), which includes Obstructive Sleep Apnea, Upper Airway Resistance Syndrome, Snoring, exaggerations of sleep induced increases in the collapsibility of the upper airway and all conditions in which inappropriate collapsing of a segment of the upper airway causes significant un-physiologic obstruction to airflow. This collapse generally occurs whenever pressure in the collapsible portion of the airway decreases below a level defined as a “critical tissue pressure” in the surrounding wall. The PAP therapy is directed to maintaining pressure in the collapsible portion of the airway at or above the critical tissue pressure at all times. In the past, this goal has been achieved by raising a pressure delivered to the patient's airway to a level higher than this critical tissue pressure at all times when the patient is wearing the device.
In general, the need for the PAP therapy occurs only during sleep. However, the conventional PAP therapy has not taken sleep/wake state into account, and conventional PAP systems apply pressure unnecessarily when the patient is awake. The applied pressure is either a constant pressure, or a pressure based on breath-by-breath determination of the need for treatment. Various strategies for determining the minimal pressure have evolved based on recognizing pathological events (e.g., apnea, hypopnea and other evidence of high airway resistance) as determined by feedback from a variety of signals that indicate the need for the PAP therapy due to the airway collapse.
Despite its success, limitations on the use of the conventional PAP systems still exist based on, for example, discomfort from the mask and the pressure required to obliterate the apneas. In particular, patients often report discomfort due to high pressure while being awake. To avoid this discomfort, the applied pressure should be provided only when the patient is asleep. For example, a “ramp” system utilizes a patient activated delay in the onset of the applied pressure, but the ramp system is not automatically responsive to patient awakenings during the night, unless deliberately activated by the patient pushing a button.
Patient's discomfort during wakefulness is often associated with changes from a regular breathing pattern (e.g., near constant breath size and frequency) to one which contains irregularities. These irregular patterns (e.g., including isolated big breaths, short pauses, and changes in breath flow shape that do not vary in any regular pattern) are recognized by inspection of the airflow tracing alone, and frequently occur when the patient is distressed by the PAP system.
Some conventional PAP systems utilize algorithms which continuously and automatically titrate the applied pressure. These algorithms depend on detecting evidence of airway collapse from the breathing signals. However, these algorithms of the conventional PAP systems have certain limitations. For example, the irregular pattern of breathing present while a subject is awake, and more so when anxious, interferes with the processing of the breath signal that calculates the applied pressure.